% ROM, % RAM

NoraANDnora · Guest

Upon compiling my code, written for PIC16F877A, I see some memory percentages.
ROM=62%
RAM=46%-100%

Where does the actual code reside?
What resides in RAM?
Do global variables reside in one place and local another?
Can I see this information in the LST file?
Which does the EEPROM section of this PIC refer to?
Which does the FLASH section of this PIC refer to?

Apologies in advance Embarassed

if this has been discussed on this forum (and I am sure it has been), I have not been able to find this discussion.
Thank you!
NN

baltazar_aquino · Joined: 16 Mar 2008 Posts: 27

The code will reside in ROM. No way will the code be read from the RAM because the program counter is designed to access the flash area only.
Constants as well resides in ROM. The RAM holds the variables. The EEPROM statistics is normally not reported by the compiler because this is usually under the control of the programmer. The flash is your ROM. Hope it answered all your questions.
Wink

Nora · Joined: 23 Mar 2008 Posts: 50

Thanks for the reply.

So how will I know how much memory I am using in EEPROM?

-N_N

Nora · Joined: 23 Mar 2008 Posts: 50

I guess I will just know how much EEPROM I'm using because of how many bytes I store?

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Nora · Joined: 23 Mar 2008 Posts: 50

So what kind of data would I be storing in EEPROM?

In my program code, I have variables that are read in from RS232 and A2D.
I have some constants.
And I have calculations that use up lots of memory (pwr with floats)- these calcs use the variables I mentioned before.

Should I store any of this in EEPROM? Where should the calculations reside? What if I need to access it quickly- then just leave it in the program code?

Thanks!
N_N

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Here a small overview of the different memory types:
- RAM is used for the variables in your program.

- ROM is used for program memory and data that never changes.

- EEPROM is used to store data that changes not very often and that you want to remember after a power loss. Examples of this are configuration and calibration data, passwords, a counter for number of device resets, serial number, etc.

A bit confusing might be that most PIC processors have Flash EPROM memory. This is the ROM memory used for storing the program. The difference between Flash EPROM and EEPROM is that Flash EPROM has a large capacity but can only be programmed in large blocks, while the EEPROM memory has small capacity but allows changing a single byte.

EEPROM is not used very often and certainly not for the variables in your calculations. EEPROM is slow and has a lot of overhead.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

What is your real problem? Is your memory full and you want to know how to optimise?

Nora · Joined: 23 Mar 2008 Posts: 50

No problem- yet. Starting a new design and trying to optimize beforehand Smile

*edit: And yes, I'm finding the difference between Flash EEPROM and EEPROM to be very confusing.
I've used an external EEPROM to store data, and am now trying to fit it all into one chip (PIC).

N_N

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Nora · Joined: 23 Mar 2008 Posts: 50

I am not aware of the technique.
Can you point me in the right direction?
Thanks!
N_N

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

A scaled integer means you use a regular integer to store decimal values. When you need the "real" value, divide the number by a correction factor. For example, if you need to store a price in the range $0.00 to $327.67, you can store it in a 2-byte int16 as 0 to 32767. You just divide it by 100 when you need to show the dollar value to the user. Or even better, don't actually divide by 100 but just insert the dot at the second decimal using the %w format specifier of printf.

Another way to phrase the above example is: Instead of calculating in dollars with two decimals (floating point) you can do the same arithmetic in dollar cents (integer).

Note that floating point data in the PIC is only exact up to the 6th or 7nd decimal (source)

As a last thing I like to re-post a very nice practical example posted some time ago on this forum:

Safvi · Guest

Its been over 7years, that i've been using CCS C compilers. I'm encountering a problem that I could not find a solution ??
Need Help......

how do I store data in the internal eeprom, as pre processor directive, that is, I donot want to declare it as constant, as it takes up Code memory. I wouold like to store it directly in the internal eeprom, and I'll access it while the program is running (with read_eeprom( X ) command )

Help needed urgently!!!

Matro · Guest

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA